Keyboard operated electronic musical instruments of the digital waveform synthesising type are well known, notable examples being U.S. Pat. Nos. 3,515,792, 3,809,786 and 3,639,913. When it is required to produce a polyphonic waveform synthesiser wherein several waveforms of different fundamental pitch, instantaneous amplitude and harmonic content are to be generated simultaneously, several options for implementation are open. One waveform generator could be assigned for each simultaneously sounded note up to some maximum number of allowable notes. This is expensive in production if the maximum allowable number is high. An alternative is to use time-sharing techniques using just one tone generator wherein each simultaneous note is given a discrete time slot in a repetitive sequence of time slots. U.S. Pat. No. 3,639,913 describes such a technique wherein the `phase-angle calculator` and the wave-shape memory are shared by each simultaneously generated tone.
Control of data flow through such a time-shared system needs to be very precise in order for the system to perform correctly. As the maximum allowable number of simultaneously sounded notes increases so the logic circuitry for producing the necessary data flow control signals also increases. The implementation of this control signal logic is specific to the particular system which is being controlled and therefore only a "random logic" array comprising S.S.I. circuits or a dedicated and inflexible L.S.I. circuit can be used. The waveform generator therefore becomes expensive either due to the high volume of S.S.I. circuits required in production or the high pre-production investment in a special purpose L.S.I. controller.